Method of operating a centrifugal filter and a filter centrifuge operating according to this method

ABSTRACT

A method of operating a filter centrifuge or centrifugal filter in which a hydrostatic head is provided at the filtering surface so as to control liquid passage therethrough. The head may promote movement of the liquid through the filter layer or may induce reverse flow of liquid for backwashing or rinsing. In addition, a hydrostatic balance may be provided at this surface through control of the head to limit passage of the liquid through the filter layer. Preferably the head provides a suction augmenting centrifugal filtration.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of Ser. No. 423,579 filedDec. 10 1973 now U.S. Pat. No. 3,943,056 issued Mar. 9 1976.

FIELD OF THE INVENTION

The present invention relates to centrifugal filters or filtercentrifuges and, more particularly, to improvements in filter centrifugestructures and especially to an improved method of operating a filtercentrifuge.

BACKGROUND OF THE INVENTION

In general a filter centrifuge or centrifugal filter comprises a rotarybasket or drum which may be driven at high speeds centrifugally to casta suspension of particulate material in a liquid phase against a filtermedium, e.g. filter cloth lining the drum. The centrifugal force drivethe liquid phase through the filter medium on which the solid phase isretained in a filter cake.

It has been proposed to increase the throughput of such filters byhydrodynamically increasing the pressure differential across the filtermedium, e.g. by evacuating liquid from the far side thereof and/orapplying a fluid pressure to the downstream side thereof with respect tothe direction of flow of the liquid through the filter medium. A systemwhich only evacuates the space around the upstream side of the drum islimited in the pressure differential which can be applied by the vaporpressure of the liquid phase.

OBJECTS OF THE INVENTION

It is an object of the invention to extend the principles set forth inapplication Ser. No. 423,579 (U.S. Pat. No. 3,943,056).

It is another object of the present invention to provide an improvedmethod of operating a centrifugal filter whereby disadvantagesencountered with earlier systems can be avoided, more effectivelyresults can be obtained with simpler filter structures, and control of afiltration process is facilitated.

It is another object of the present invention to provide a centrifugalfilter of simple construction and facility of control, which structurealso is characterized by a throughput controllable as desired.

SUMMARY OF THE INVENTION

These objects and others which will become apparent hereinafter areattained, in accordance with the present invention, in a method ofoperating a filter centrifuge, especially for the filtration ofsuspensions to separate a liquid phase from the solid phase, in which asuction effect is provided at the filter medium and wherein the filtercentrifuge is provided with a liquid-collecting compartment having atleast one discharge opening.

According to the invention, radially outwardly of the filter mediumthere is provided a liquid column or head which generates a hydrostaticpressure acting at the filter cake and controllable to regulate thepressure differential across the filter medium and/or the filter cake.

While the hydrostatic head can also be provided by a controlled pumpingaction or by throttled outflow, it is preferred to form a furthercompartment in fluid communication with the liquid-collectingcompartment through the aforementioned discharge opening or openings sothat a level difference (as measured radially from the axis), exists inthe two compartments to establish the liquid head. According to theinvention, therefore, a hydrostatic head, as distinct from thehydrodynamic pressure differential used heretofore, is controlled toadjust the ratio of the pressures on opposite sides of the radiallyouter surface of the filter cake and thereby regulate the passage ofliquid through the filter cake and the filter medium or create acondition in which liquid passage therethrough is at standstill.

The system just described does not, therefore, utilize the hydrodynamicsuction effect of conventional systems to induce liquid transport acrossthe interface between the filter medium and the deposited solids, butrather relies upon a hydrostatic effect (siphon effect) at thisinterface or the so-called displacement principle to induce thetransport of liquid across the interface. In other words, the process ofthe present invention utilizes the static component is of the BERNOULLIequation compared to which the dynamic component is negligible.

The utilization of the hydrostatic suction effect allows correspondinglyhigher filtrate throughputs to be achieved such that a suction of about10 meters (water column) below atmospheric and corresponding to thevapor pressure of the liquid used is attainable. Such suction levelshave not even been approximated with prior-art filter centrifugeswithout enormous expenditure.

According to another feature of the invention, prior to the mainfiltration, air is eliminated from the collecting compartment and isexcluded therefrom by the liquid present therein and in the centrifugedrum. Thus the present invention further comprises the step of expellingair from the collecting compartment preferably by the backflow of liquidinto the latter from the other compartment of the liquid head. The airis thus expelled through the filter medium and a liquid continuum iscreated at least from the interface between the filter medium and thecollected solids to the free liquid surface of the other compartment ofthe liquid column.

Throughout this description, reference will be made to a liquid head ora liquid column, terms which are commonly used in systems in which theliquid is confined in a column as in a manometer. This terminology hasbeen adopted here notwithstanding the fact that the collectingcompartment in an annular chamber and that the other compartmentcommunicating therewith and axially offset from the liquid-collectingcompartment is also generally an annular chamber.

Since the present system is concerned with a centrifugal arrangement,the free surface of the liquid in the other compartment will lie at adistance R from the axis of rotation of the drum. The aforementionedinterface may lie at radial distance r from the axis of the drum so thatR = R + h, where h is the radial difference (R - r) and corresponds tothe difference between liquid levels in a U-type manometer tube.

While in a manometer tube the difference between liquid levelsrepresents a pressure differential or head depending upon the liquiddensity and the gravitational force at the location of the manometer, ina centrifugal system the head is a function of this dimension h, theliquid density and the centrifugal acceleration. Hence the distance hrepresents a true liquid head and the continuum of liquid from theinterface to the free surface constitutes a hydrostatic liquid "column"generating this head.

In the system in which the free surface of the other compartment liesradially outwardly of the interface between the filter medium and thecollected solids and a continuum of liquid is established between thisinterface and the free surface, the head h is a suction head tending todisplace liquid through the filter cake and the filter medium. When thefree surface lies radially inwardly of the interface, the head tends todisplace liquid in the opposite direction (from the exterior inwardly)through the filter medium to drive air therefrom to commence siphonaction. The substantial exclusion of air is thus necessary in theestablishment of the liquid continuum upon which the hydrostatic effectpreviously described is based.

It should, of course, be noted that since the "head" involved in thepresent case is a function of centrifugal action, orientation of thecentrifuge drum is not material except as it may affect the manner inwhich the liquid continuum is maintained.

According to another feature of the invention the liquid is notdischarged from the collecting compartment during the discharge of airtherefrom.

It has previously been mentioned that the liquid head for displacing theliquid through the filter medium of the centrifuge is constituted as thedifferential head of two interconnected liquid columns. In the case inwhich both these columns have equal heights (measured along the radius),the system resembles a U-tube with equal-length arms such that thehydrostatic pressure differential is eliminated and the only filtrationwhich can occur is that brought about by pumping action.

The levels of the two liquid columns which generate the resultantpressure differential or head mentioned previously, should mostadvantageously be adjusted radially externally of the filter cake. Thisadjustment can be carried out so that the pressure differentialapproximates the vapor pressure of the liquid so that a further dropbelow the vapor pressure of the liquid results in a transformation of aportion of the liquid column to the vapor phase. The differential headis preferably steplessly adjustable between the vapor pressure of theliquid (maximum filtration) and a negative pressure differential ofseveral atmospheres (corresponding to the backflow of liquid) through apressure differential which, in dependence upon the centrifugal action,brings about standstill of the liquid phase of the interface.

A reduction of the filtrate throughflow velocity is desirable, forexample, upon changeover from filtration to washing under conditions inwhich the penetration of air into the filter cake is to be avoided. Aprotracted filtration (low filtrate flow rate) is also desirable forsolvent washing, extraction processes and similar techniques.Furthermore, it is frequently desirable to fill this centrifuge duringan interlude of low or throttled filtrate flow rates to obtain a uniformdistribution of the solids over the interior of the centrifuge drum andwithout reducing the speed of the drum as has been required heretofore.

To exclude air from the system and establish the liquid continuum, aliquid is initially introduced through the discharge compartment and theopening or openings connecting same with the liquid-collectingcompartment, i.e. by an initial reverse flow of liquid toward the filtermedium from the exterior of the drum. This drives any air trapped in thecollecting compartment through the filter cake and it has also beenfound to be desirable to force a portion of this liquid through thefilter cakes or the filter medium. A reverse flow of this type may beused whenever air accumulates in the collecting compartment andthreatens to interrupt the liquid continuum or whenever a washing of thecollected solids is required or rinsing (back-washing) of the filtermedium is desirable.

According to another feature of the invention, externally of theliquid-collecting compartment and preferably in the centrifuge housing,there is established a controllable superatmospheric pressure with agaseous medium. Under these circumstances the effective filtrationpressure can be the sum of the superatmospheric pressure applied to thecentrifuge housing and the head developed by the liquid columns. Thesuperatmospheric pressure can be, for example, two bars. This canincrease the effective hydrostatic suction head to about 20 meters(water column as calculated at the acceleration of gravity) which wouldcorrespond to an acceleration of 500 G at the drum with a radial head hof about 40 mm.

According to another important feature of the invention, during the mainfiltration process, only sufficient liquid is withdrawn through thedischarge opening that a surface of the suspension is formed radiallywithin the filter medium and is there maintained.

It will be appreciated that normal filtration generally operates withthe approach that the liquid phase (filtrate) should be drained asrapidly as possible so that the suspension (solid particles dispersed inthe liquid phase) has no free or continuous surface within the filtermedium or inwardly thereof in a filter centrifuge unless the latter isnot operating with a sufficient throughput.

However, the present system operates most effectively when a liquidcontinuum is maintained within the liquid-collecting chamber orcompartment and up to at least the interface between the filter mediumand the accumulated and collected solids forming the filter cake uponthe filter medium.

It has been found that the effect of this continuum is augmented bylimiting the rate at which the liquid phase is drawn through the filtermedium so that radially inwardly thereof the continuum is maintained bythe suspension of solid particles in the liquid phase. This ensures thatthe "displacement"-type suction commences within the filter cake orimmediately adjacent the latter and that air will not penetrate into thefilter cake or into the liquid-collecting compartment to interrupt theliquid continuum prematurely.

Advantageously, to choke off the filtration (passage of filtrate throughthe filter medium) the two liquid columns to either side of theirconnecting passages are adjusted so that the pressures on opposite sidesof the radially outermost surface of the filter cake are approximatelyequal. This can be obtained by increasing the radial height of theliquid in the axially outermost compartment, thereby reversing thepressure head to balance the head of the liquid within the drum asmeasured radially inwardly from this outermost surface of the filtercake.

With this technique, the filtration can be brought completely tostandstill, a possibility which is signficant not only for theinterchange from a filtration operation to a washing operation, but alsowhen fresh suspension is not supplied and an interruption of the liquidcontinuum is undesirable. For example, assume that the supply of thesuspension to be filtered is interrupted. The liquid phase previouslyforming the suspension would otherwise traverse the filter cake, thefilter cloth and even the liquid-collecting compartment. Beforefiltration is commenced anew, it would be necessary to drive out the airwhich has thus penetrated as previously described for the initial stepsin the filtering process. When, however, hydrostatic balance is achievedat the outer surface of the filter cake, i.e. at the interface betweenthe filter medium and the filter cake, the throughflow of the liquidphase can be terminated with a layer of liquid overlying the filter cakeor at least the filter medium so that air penetration is not a problem.

It has been found to be desirable that, upon termination of the mainfiltration, the pressure radially outwardly of the filter cake is soadjusted that a reduction of the level of the suspension surface withrespect to the filter cake is effected in a progressive manner, untilfinally the surface of the liquid lies at the radially outer surface ofthe filter cake. This progressive draining of the filter cake under thehydrostatically controlled pressure promotes drying of the filter cake.

According to the apparatus aspects of the invention, the filtercentrifuge of the present invention can comprise a housing enclosing afilter drum which is rotatable about an axis and is formed with aperforated or otherwise liquid-permeable supporting surface centered onthe axis and adapted to carry the filter medium, e.g. a layer of filtercloth lining the interior of the drum. The drum is formed, outwardly ofthis liquid-permeable surface, with the liquid-collecting compartmentwhich communicates at a location radially spaced from the filter mediumwith the discharge outlet.

According to a feature of the invention, a wall lying in a planeperpendicular to the axis forms the base of the drum and is, in turn,formed with the discharge apertures which open into the aforementioned"other" compartment axially offset from the liquid-collectingcompartment and likewise annularly extending around the axis of thedrum. The two compartments are thus disposed on opposite sides of thewall. Means is provided for adjusting the level of the liquid in thisother compartment which, together with the liquid-collecting compartmentform a pair of liquid columns having liquid levels which may lie atdifferent radial distances from the axis to define the hydrostatic headmentioned previously. The term "level" is here used to describe theposition of the liquid surface in the compartment as centrifugallycreated and disposed at the desired distance from the axis.

In the system claimed in the above-mentioned prior application it isrequired that the drum axis be horizontal. However it has been foundthat, while the horizontal axis is preferable in some cases it is notessential and the centrifugal operations allow a vertical axis to beused as well.

According to a particularly advantageous feature of the invention, theoutlet openings are connected with a siphon arrangement which may beconstituted by the annular liquid-collecting compartment, the dischargeopenings and the annular discharge compartment which is likewiseconcentric with the axis of rotation of the centrifuge and whichcommunicates with the discharge openings by inlet openings in the wallof the discharge compartment.

When the inlet and discharge openings are at the same distance from theaxis of rotation of the drum, they may simply be opposite sides of boresformed in the wall defining the base of the drum and separating the twocompartments from one another.

In another construction, however, whereby the head can be greater thanin the embodiment just described, the discharge or second compartment isdisposed radially outwardly of the liquid-collecting compartment and theinlet openings are connected with the discharge openings by tubes.

The liquid-removal means preferably constitutes a skimmer tube whoseskimming end or mouth can adjustably be disposed in the region betweenthe distance of the filter medium from the axis and the distance of theinlet openings therefrom. Preferably the skimmer end is displaceable byswinging the skimmer tube about an axis parallel to the axis of rotationof the drum and advantageously disposed between the skimmer mouth andthe drum axis.

With this embodiment it is possible to permit the liquid level in theannular discharge chamber to have a greater radius than the liquid levelin the liquid-collecting compartment (as measured from the axis of thedrum) and thereby creates a negative head, i.e. a hydrostatic headfavoring passage of the filtrate through the filter cake and the filtermedium. The head corresponds to the product of the difference betweenthe radii of the liquid level, the centrifugal acceleration and thedensity of the liquid. The difference between the radii corresponds by asimilar arithmetic procedure to the hydrostatic differential between thepressure in the interior of the centrifuge drum and the reduced pressurein the liquid-collecting chamber.

This reduced pressure can be lowered to a value corresponding to thevapor pressure of the liquid at the controlling temperature and isautomatically maintained by the centrifuge according to the presentinvention using the combination of a siphon and a liquid skimmer. Ofcourse, the pressure differential across the filter cake and filtermedium also depends upon the supply of liquid and the flow of liquidfrom the filter cake.

With a constant liquid level (radial) in the discharge part of thesiphon device, for example, by an appropriate adjustment of the mouth ofthe skimming tube, the throughflow of liquid traversing the perforatedwall of the drum into the liquid-collecting compartment is maintainedconstant even as the solids accumulate on the inner wall of the drum.The liquid flow across the filter medium is thus equal to the liquidremoved over a given time span from the discharge compartment of thesiphon. The suction is maintained at the filter medium until influx ofthe suspension terminates and liquid is drained from the system so thatair begins to enter the filter cake. Between the commencement offiltration and this interruption of the liquid continuum, the suctionfiltration is automatic and self-regulating without any special need orexpense for control.

The separation of suspensions with the filter centrifuge of the presentinvention thus can be designated primarily as a pressure-stagefiltration.

According to a feature of the invention, the inwardly open outercompartment, which may be generally trough-shaped, can be formed with aplurality of openings at different distances from the axis of rotationof the centrifugal drum and selectively openable to adjust the radiallevel of the liquid in this outer compartment. Thus, after liquidcompletely fills the liquid-collecting compartment and has reached acorresponding level in the outer compartment, a port radially spacedfrom the axis by a distance greater than the radial spacing of thesuspension surface within the drum, can be opened to generate ahydrostatic head promoting suction through the filter medium.

Alternatively, or in addition, the outer compartment can be providedwith an inwardly extending annular apron lying in a plane perpendicularto the axis and having an inner edge which lies radially outwardly ofthe filter medium to form a weir across which the liquid can flow inaccordance with siphon principles. When this weir arrangement is used,the liquid can be deflected through a further bend to increase thesiphon force or form a trap preventing the influx of gas into thesystem.

It has also been found to be advantageous in some cases to provide asimple outlet or construction for the liquid-collecting compartmentwhich may be formed by a throttle valve or the like designed to have thesame effect as the outer compartment mentioned earlier. In this case,the outlets from the liquid-collecting compartment must be disposed at agreater radial distance from the axis of the drum than the filtermedium.

According to still another feature of the invention, means is providedfor evacuating air or other gases from the space around the filtermedium, i.e. the liquid-collecting compartment, to facilitate filling ofthe latter with liquid whether from an external source or from thefiltrate of the suspension which is to be separated. To this end,passages may be provided in a wall of the liquid-collecting compartmentand can open radially outwardly of the filter medium, these passagesbeing connected to a suction pump built into the drum or remotetherefrom. Alternatively, a check valve may be provided to permitunidirectional displacement of air out of the system by the filtrate ora siphon-starting liquid.

It has also been found to be advantageous in some cases to provide theliquid-collecting compartment between a fixed housing structure and therotatable drum and, for this purpose, the drum may be provided with oneor more radially extending annular projections which extend into annularpockets of the fixed housing or merely subdivide the fixed housing intothe liquid-collecting compartment or the compartment from which liquidis drained to sustain the siphon effect.

In order to reduce the frictional effect of relative rotation of theliquid and the rotating drum or the stationary housing, means may beprovided along the exterior of the drum to effect entrainment of theliquid in the collecting compartment or the liquid-collectingcompartment itself may be reduced in axial width by providing it betweena pair of inwardly extending axially spaced annular partitions flankinga pair of outwardly extending annular vanes disposed inwardly of the endof the drum.

In accordance with still another feature of the invention, theliquid-collecting compartment is provided on the drum and the latter hasa centrifugally closed valve which prevents escape of liquid until theliquid level (in the radial sense) builds up sufficiently to sustain asiphon-type suction at the filter medium. The valve can have a flowwhich responds to the liquid level to open the outlet and permitthrottled drainage of the liquid-collecting compartment.

In general, the centrifuge can have either a vertical or a horizontalaxis of rotation and the embodiments described hereinafter may be shownto have one or the other, but if shown to be horizontal can also bevertical. The liquid-collecting compartment will extend in either casecircumferentially around the filter medium which may be a filter clothsupported on a perforated plate, grate or the like. While it ispreferred to have the liquid-collecting compartment increase in crosssection toward the outer compartment, it is also possible to constitutethe liquid-collecting compartment only of the interstices of the grate,grind or the like supporting the filter medium. In the latter case tubesor the like can connect the liquid-collecting space with the outer spacedisposed radially outwardly of the filter medium. The drum may also beformed with a plate subdivided the liquid-collecting compartment fromthe outer compartment and providing the clearance radially outwardly ofthe filter medium through which liquid communication is established orformed with bores establishing such communication.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing in which:

FIG. 1 is a longitudinal section, somewhat in diagrammatic form, in acentrifuge according to the present invention;

FIG. 1A is a sectional view taken generally along the line IA -- IA ofFIG. 1;

FIG. 1B is a section similar to FIG. 1A;

FIG. 2 is a section of a fragement of a centrifuge according to anotherembodiment of the invention but with functional chracteristics similarto those of the centrifuge of FIG. 1;

FIG. 3a is a diagrammatic section of a centrifuge with a vertical axis;

FIG. 3b is a diagrammatic longitudinal section illustrating anadditional centrifuge configuration according to the present inventionand although having a horizontal axis of rotation may be disposed with avertical axis;

FIG. 3c is a view similar to FIG. 3b but illustrating anotherembodiment;

FIG. 3d is another similar view;

FIG. 3e is still another view of this type;

FIG. 4 is an axial cross-sectional view through a filtercentrifuge drumprovided with an air-suction device for drawing air from theliquid-collecting compartment;

FIG. 5 is an axial cross-sectional view through a filter-centrifuge drumprovided with another air suction device and a closure for theliquid-collecting compartment;

FIG. 6 is an axial cross-sectional view through a filter centrifugeprovided with another air-suction arrangement for filling the siphon;

FIG. 7 is an axial cross-sectional view of a filter centrifuge drum inwhich the air-withdrawal device likewise constitutes the system fordraining the liquid phase;

FIG. 8 is an axial cross-sectional view through a drum of centrifugalfilter in which the siphon is provided with means for increasing thehydrostatic suction force;

FIG. 9 is an axial cross-sectional view through a filter centrifuge drumprovided with other means for removal of gas, e.g. including checkvalves;

FIG. 10 is an axial cross-section view through a portion of a filtercentrifuge drum representing a modification of the system of FIG. 7;

FIG. 11 is a view similar to FIG. 10 but representing anothermodification;

FIG. 12 is an axial cross-sectional view of the siphon portion of afilter centrifuge drum having an additional suction device;

FIG. 13 is a view similar to FIG. 12 and like FIG. 12 represents amodification of the system of FIG. 5 in which the suction device isformed by a liquid jet pump or ejector; and

FIG. 14 is an axial cross-sectional view through the lower portion ofanother filter centrifuge drum having a floatcontrolled dischargeopening for the liquid-collecting compartment.

SPECIFIC DESCRIPTION

In the following description all embodiments with the possible exceptionof those of FIG. 1, 1A, 1B, 2, 3c, 7, 8 and 14 can operate effectivelyin rotation about vertical or horizontal axes.

In FIGS. 1, 1A, 1B and 2, similar reference numerals have been used todesignate similarly functioning parts. In these Figures, there has beenillustrated a centrifugal filter having a filtering drum 1 which isformed with a perforated cylindrical shell or wall 2 mounted upon ashaft 3 for rotation about the horizontal axis of this shaft. The drum 1is formed with a jacket 4 which defines a liquid-collecting compartment5 radially outwardly of the layer 7 of solids collected along the innerwall of this drum. In the embodiment of FIG. 1 and FIGS. 1A and 1B thisliquid-collecting compartment increases the cross-section axially towardthe base of the drum and in both FIG. 1 and FIG. 2, theliquid-collecting compartment 5 reaches substantially to the radiallyoutermost surface of the collected solids. Thus in the embodiment ofFIG. 1, the liquid-collecting compartment 5 includes the perforations ofthe cylindrical wall 2.

The liquid-collecting compartment thus represents the totality of all ofthe interstices of the filter medium, e.g. a filter cloth 6 and all ofthe openings and passages of the perforated plate or grate supportingthis filter medium. Any further spaces radially outwardly of the mediumand communicating with the supporting perforated plate or grate likewiseis considered to constitute part of the liquid-collecting compartment.

The filter medium 6, which has been illustrated diagrammatically as afilter cloth but may be a preformed layer of filter particles or anyother medium permeable to the liquid phase whose flow cross-section issuch as to prevent passage of the solid phase, rests upon the perforatedshell 2 in the embodiment of FIG. 1 and, in turn, supports the filtercake 7 of collected solids.

The base of the drum (FIG. 1) is formed with a wall 8 which separatesthe liquid-collecting compartment 5 from a discharge compartment 11which is disposed in axially offset relationship with respect to theliquid-collecting compartment 5.

The compartment 11 is thus defined between the drum base 8 and anannular wall or apron 9 lying parallel to the drum base 8 in a planeperpendicular to the axis of rotation of the drum. A narrowaxially-extending cyclindrical wall 10 bridges the walls 8 and 9 andthus constitutes of the compartment 11, an inwardly opening annulartrough for which the wall 9 functions as a weir. The floor 10 of thecompartment 11 lies radially outwardly of the perforated wall 2 of thedrum and hence the extreme outer face of the filter 7.

In the region of its largest diameter, the liquid-collecting compartment5 is formed with a plurality of individual discharge opening 12 whichcommunicate with inlet openings 13 in the drum wall 8 in the region ofthe largest diameter of the annular compartment 11, i.e. in the regionof the trough bottom 10.

A swingable skimmer tube 14 projects downwardly into the trough-shapedcompartment 11 and has a tube end 15 so adjustable that it may be movedfrom an inner position close to the radial distance of the inner filtercake surface from the axis to a position close to the regions of theopenings 13. The range of mobility has been illustrated in FIGS. 1A and1B from which the configuration of the skimmer tube can also be noted.The tube end 15 opens against the sense of rotation of the drum so thatthe liquid in the trough-shaped compartment 11 rotationally entrained bythe drum, for example, in the counter-clockwise sense A as representedin these Figures, it carried into the oppositely open end 15 of the tube14 which, in turn, carries the liquid away to maintain the liquid levelin compartment 11 substantially at the position of the mouth 15. Thetube 14 is mounted upon a horizontal pipe 14a which is swingable in aseal 14b about a horizontal axis B parallel to the horizontal axis C ofrotation of the drum. A supply device 16 serves to introduce thesuspension into the filter drum 1. This supply device may comprise aduct 16a formed with a control valve 16b for establishing the flow ofthe suspension to the drum and a distributing head 16c for spraying thesuspension substantially uniformly over the axial length of the innerwall 2 of this drum.

A scraping arrangement 17 is provided to remove the collected solidsfrom the filter cake 7 and comprises an adjusting element 18. Thescraper arrangement and the adjusting element are of known configurationand can include a blade 17a at the upper edge of a hopper 17b whichcommunicates with a chute 25 leading from the housing 20 of thecentrifuge. A valve 25a diagrammatically represents means for blockingthe chute during the filtering process to prevent loss of pressurewithin the centrifuge.

In the system illustrated in FIGS. 1 and 2, a filling tube 19 isprovided for delivering liquid as controlled by a valve 19a to thetrough-shaped compartment 11 for starting the siphon action and drivingair from the siphon arrangement. Of course, this function may also beperformed by the skimmer tube 14 with appropriate repositioning of thevalve 19a to communicate therewith.

The housing 20 encloses the centrifuge drum and hermetically seals thelatter. A pipe 21, connected via valve 21a or another suitable controlelement, can communicate with a compressor or source of inert gas (i.e.a tank of nitrogen or argon) to pressurize the centrifuge. The pipes,ducts, etc. 16, 17, 14, 18 and 19 as well as the shaft 3 are in theusual manner sealed with respect to the housing 20. A seal for thispurpose has been shown at 22 which hermetically retains the pressurewithin the housing 20 but permits rotation of the shaft 3. The apparatushas a conventional drive (not shown) for the latter shaft.

In the apparatus of FIG. 2, the liquid-collecting compartment 5 differsfrom that of FIG. 1 in that it consists only of the relatively smallspace around a supporting grate 23 for the filter cloth 6. The grate 23may be made of bars or wires running circumferentially along the innerwall of the drum shell 26 and carrying the filter cloth. Of course, agrid, screen or like arrangement may also be used.

In the embodiment of FIG. 2, moreover, the outlet openings 12 are formeddirectly in the cylindrical wall of the shell 26 and are connected byshort bent tubes 24 with the inlet openings 13 of the annular dischargecompartment 11. In this embodiment, as in the embodiment of FIG. 1, askimming device 14 of adjustable penetration into the trough-shapedcompartment 11 is employed.

The system of FIGS. 1, 1A, 1B and 2 operates substantially as follows:

At normal (ambient) pressure within the housing 20, liquid is admittedby the filling pipe 19 into the compartment 11 of the drum which isrotated at its normal centrifugal filtration speed. No suspension is tobe found within the drum and no filter cake overlies the filter cloth.Liquid (water) is admitted in this fashion until the level in theliquid-collecting compartment 5 rises to the filter medium 6 or extendsto a small distance radially inwardly thereof. During this procedure,all of the air is driven out of the liquid-collecting compartment 5. Themouth 15 of the skimmer tube 14 is, during this period swung radiallyinwardly to the maximum extent permitted by the structure (see FIG. 1B).

Using the feed device 16, the suspension of solid particles in liquid(e.g. water) is introduced into the interior of the drum and iscentrifugally distributed uniformly over the interior of the drum. Thefiltrate tends to displace the previously introduced liquid from theliquid-collecting compartment 5 until its level within the interior ofthe drum 1 and the level in the annular compartment 11 are identical.

The flow of filtrate through the system is then accelerated by swingingthe mouth 15 of the skimmer tube 14 radially outwardly as quickly aspossible, the liquid being driven into the mouth of the tube and out ofthe system by the pumping action of the drum. Since this immediatelylowers the liquid level in the outer compartment 11 to that determinedby the setting of the mouth of skimmer tube 14, a hydrostatic pressuredrop is applied across the liquid column between the mouth of theskimmer tube 14 and the surface of the liquid within the drum. A similarsuction can be measured between the outlet opening 12 and the inletopening 13. The suction force augments the filtration by centrifugalaction.

The suction force is, as previously noted, a function of the differencein the levels of the liquid in the annular compartment 11 and theliquid-collecting compartment 5, representing the reduced pressuremanifested in an upright-tube manometer.

In this case, the liquid level in the liquid-collecting compartment 5can be represented at r and is considered to lie just at the filtercloth while the liquid level in the outer compartment is considered tobe at R, both distances being measured from the axis B of rotation ofthe drum. As a consequence, the head is a function of the difference h =(R - r) and, of course, is also a function of the centrifugalacceleration and the density of the liquid.

For continuous operation, the suspension is permitted to flowcontinuously into the drum by the feed arrangement 16 and the skimmertube 14 is maintained in the position shown in FIGS. 1 and 1A so that aliquid continuum maintains the syphon over the entire filteringinterval. More generally and most advantageously, the suspension isintroduced intermittently into the drum and the above-described sequenceof steps is repeated for each fresh introduction of the suspension, i.e.the skimmer tube is swung rapidly into the trough 11 for each freshaddition of suspension to accelerate the filtration.

The reduced pressure brought about by the siphon effect generallycannot, of course, lie below the vapor pressure of the liquid at theoperating temperature although it can reach this pressure by merelyappropriately dimensioning the radii of the liquid-collectingcompartment 5 and the annular trough 11 and with an appropriate removalof liquid from the latter.

The suction effect can be augmented when, where a filter cake 7 ispresent in the centrifuge, the liquid introduced at 19 is first forcedthrough the filter cake and preferably has a level slightly inwardlythereof upon startup of the centrifuge. In this case, the filtrate israpidly drawn by the siphon effect through the filter cake as well asthe filter medium. The air trapped within the filter cake is therebyfirst displaced by the backflow of the siphoning liquid.

Upon commencement of filtration, the mouth of the skimmer tube 14 isswung inwardly and simultaneously the pressure in housing 20 can beincreased so that the gas pressure differential across the siphon isaugmented at the same time as the siphon liquid is accelerated byextraction of liquid from the annular trough 11. In this case, theannular trough 11 may be held under atmospheric pressure while theelevated pressure is applied at the interior of the drum. Of course, inthe embodiment illustrated in FIG. 1, the interior of drum 1 and theannular compartment 11 are both exposed to the same elevated gaspressure and this pressure serves to drive fluid out of the trough 11through the skimmer tube 14. The pressure may be provided in the form ofan inert gas atmosphere in accordance with the principles ofpressure-stage filtration.

In FIGS. 3a - 3e there are illustrated various modifications of a filtercentrifuge according to the present invention.

In FIG. 3a, for example, the centrifuge drum, which has been illustrateddiagrammatically, comprises a liquid-collecting chamber 30 surroundingthe filter medium 36 and communicating by apertures in the base of thedrum with a trough-shaped annular compartment 33. These apertures liealong the outer periphery of the drum and are constituted as simplebores whose side turned toward the liquid-collecting compartment 30 isrepresented as outlets 31 while the side turned toward the annulartrough 33 is represented as inlets 32. The annular compartment 33 isformed with a wall 34 functioning as a weir, the weir edge 35 beingdisposed outwardly of the filter medium 36. In other words, the radialdistance between the vertical axis of rotation of the drum and theoverflow edge 35 of the weir 34 is greater than the radius of the filtermedium 36 but smaller than the spacing of the inlet openings 32 from theaxis. Since the edge 35 lies outwardly (radially) of the filter medium,the siphon effect induces continuous suction until the liquid level inthe collecting compartment 30 moves outwardly to the level of this edge.The system of FIG. 3a can thus operate without a skimmer tube.

In FIG. 3b, there has been illustrated a system of the type generallydiscussed in connection with FIGS. 1A and 1B, and in which a skimmertube 37 has a swingable mouth or end 38 which can penetrate into theinwardly open circumferential trough 33 in the operative position ofthis tube. In an inoperative position, however, the mouth of the skimmertube may be swung inwardly beyond the overflow edge 35 of an apron 39forming a weir. In this embodiment, the edge 35 lies somewhat inwardlyof the filter medium 36. The other elements of the drum of FIGS. 3b bearreference characters identical to the corresponding elements of FIG. 3a.In operation, the weir 35, 39 is effective during initial filling of theapparatus with the suspension and until the level of the suspensionwithin the drum has reached the level (radial) of the edge 35. At thispoint, the skimmer tube 37 may be swung about its axis (parallel to theaxis of the drum) to bring its end 38 rapidly into the trough 33 toincrease the suction action as described in connection with FIGS. 1 and2. The radial distance between the edge 35 and the axis may coincidewith the innermost surface of the filter cake overlying the filtermedium 36 in the maximum thickness of the filter cake. Preferably,however, the radial distance of edge 35 from the axis of the drum isapproximately equal to, or slightly greater than the radial distance ofthe filter medium 36 from this axis.

In the modification of FIG. 3c, the liquid can be drawn off from theannular trough 33 through bores spaced apart at different radialdistances from the axis, thereby eliminating the need for a skimmer tubealthough the skimmer tube as described in conjunction with FIGS. 1, 1A,1B, 2 and 3b can also be used to supplement these bores. The bores 40are formed in the apron or wall 39 defining the annular compartment 33and are provided with plugs which have been diagrammatically illustratedat 30a in FIG. 3c. These plugs each carry stems 40b which are turnedinwardly and are received in U-channels 40c of actuating members 40d sothat the stems 40b may freely rotate and can be axially withdrawn (arrowD) to open the respective bores. The bores 40, at different distancesfrom the axis, may be selectively opened or closed to establish thedesired level of liquid in the trough 33 and hence the suction effect. Afilling tube 42, analogous to the tube 19 described in connection withFIGS. 1 and 2, may be provided to supply liquid to the trough 33 todrive out air from the system and to begin the siphon action. Theinnermost edge of the apron 39 here preferably lies inwardly of thefilter medium 36 at least to the extent illustrated and described alsoin connection with FIG. 3b.

In the embodiment of FIG. 3d, the siphon effect is similar to thatgenerated in FIG. 3a, i.e. derived from the fact that the overflow edgeof the weir formed by the apron lies radially outwardly of the filtermedium 36. In this embodiment, the outlet compartment 45 is annular andcommunicates via circumferentially spaced siphon tubes 43 whosedischarge and inlet portions are represented at 31 and correspond to thesimilarly designated elements of the embodiments of FIGS. 3a - 3c. Eachof these siphon tubes 43 is provided with a closure member 44, e.g. abutterfly valve, to prevent a premature draining of liquid from thesiphon and entry of air into it when the suspension supply to the drumterminates or the liquid level within the drum falls toward the leveldefined in the weir. The valve members 44 may be actuated automatically,e.g. by a float system as will be described in greater detailhereinafter, or by some operator-controlled actuating arrangement asdescribed in conjunction with FIG. 3c. By preventing liquid drainingbeneath the inner surface of the filter cake, penetration of air intothe latter, into the filter medium and into the liquid-collectingcompartment can be precluded and the time required for driving air fromthe system and re-establishing the liquid continuum can be conserved.

FIG. 3e diagrammatically represents an arrangement in which the closuremembers or valves 36 are spring-biased axially movable plates whichnormally block the openings 31 unless a pressure head favoring thesuction effect is produced. When the suction head is sufficient, thesevalves are displaced to permit flow of fluid in accordance with thesiphon principle, the springs of the valves having adjustable strengthsto permit the resulting throttling effort to be varied at will. Theadjustability is represented by arrows in FIG. 3e and can beaccomplished with force transmitting or control rods or similar membersas described in connection with FIG. 3c or by any conventionalcentrifugal governor arrangement. The siphon system is representedgenerally at 47 and includes the outlet sides of the closures 46. Theclosures 46 are immersed in the trough or pocket formed by this siphon.This system also limits penetration or air into the liquid continuum.

FIG. 4 shows schematically an embodiment of the invention in which thedrum 51 is rotatable about a horizontal axis on its shaft 52 and isprovided with a support (not shown) which may be of the type describedin connection with FIGS. 1 or 2 for a filter medium 53. Preferably thefilter medium is a filter cloth overlying a support grid. The filtercloth carries the filter cake 54. Radially outwardly of the filtermedium 53, the drum 51 is formed with a liquid-collecting compartment 55communicating via openings 56 with an annular trough-shaped compartment57 beneath the right of the bottom plate of the drum. The liquid(filtrate) is discharged from the latter by a skimmer tube 58 which maybe of the type described in connection with FIGS. 1A, 1B and 2,swingably mounted so that its mouth may be introduced into the trough 57to vary the level 60 of liquid therein. The swingable skimmer tube isdesigned to maintain a siphon effect between the level 60 in compartment57 to provide the suction effect at the interior of the drum. A pump 63can be connected via an opening 62 above the level 59 of liquid incompartment 55 to draw air from the system in a manner to be discussedmore fully below.

Similarly, the drum 101 of the filter centrifuge of FIG. 5 carries thefilter cake 104 upon the supported filter medium 103 and is formed witha liquid-collecting compartment 105 outwardly of this filter medium. Theoutlets 106 of this compartment are provided with throttle valves 111generally similar to the valve 46 described in connection with FIG. 3e.The closures 111 thus permit throttle outflow and maintain a siphoneffect under the centrifugal action of the drum. Air can be drawn fromthe space immediately below the filter medium by a pump 113communicating via a passage 112 with the liquid-collecting compartmentimmediately outwardly of the filter medium.

FIG. 6 shows another system for evacuating air from theliquid-collecting compartment immediately outwardly of the filtermedium, such means including a suction pump 64 connected via a pipe 66to a plurality of radial ducts 65 which communicate with passages 62opening into the liquid-collecting compartment immediately outwardly ofthe filter medium and any support provided therefor.

In the embodiments of FIGS. 4 - 6, the residual air can be eliminated bythe pumps 63, 64 and 113 shown in the drawing to establish the liquidcontinuum and create or maintain the hydrostatic siphon-type suctioneffect.

The embodiment of FIG. 7 comprises a centrifuge drum consisting of abase plate 217 carried upon the shaft formed with a cylindrical support(not shown) for a filter medium 206 upon which the filter cake 204 isdeposited in the manner previously described. The housing 218 has abackplate 218a and is formed with a trough-shaped annular pocket 218binto which the plate 217 extends to define a siphon arrangementrepresented at 220. When the drum 201 also carries a front plate 217aformed with an opening 217b through which the suspension may beintroduced, this plate likewise extends with clearance into a pocket218c at the front end of the otherwise cylindrical housing 218. Thelatter is provided with a mouth 218d, registering with the opening 217bthrough which the suspension may be introduced and the solids may beremoved as described generally in connection with FIG. 1. Consequently,a siphon pocket 220a is also formed at the front end of the drum. Liquidcan be discharged from the system via a valve 221 which has beenschematically illustrated and forms a siphon arrangement as in theembodiment of FIG. 4 or has the function of the throttle valve of FIG.5. The siphon effect produces a suction which promotes filtration in themanner previously described.

In addition, the fixed housing wall is provided with a discharge opening222 which can communicate with a pump 223 by which air and/or liquid isdrawn from the liquid-collecting compartment. Consequently, no separatefilling liquid is required to establish the siphon effect since, afterthe suspension has been introduced into the drum, pump 223 may beoperated to draw liquid into the liquid-collecting compartment until thelatter is completely filled thereby displacing any air from the system.The pump has the further advantage in this embodiment, in common withthe systems of FIGS. 4 - 6 that, following the filtration and completedraining of filtrate, air or other gas can be drawn through the filtercake to dry the latter.

FIG. 8 represents a simplified version of the filter centrifuge in whichthe base plate 317 is carried by the shaft 302 and is formed with a gridrepresented generally at 317a and supporting the filter medium 303. Thelatter carries the filter cake. The grid 317a is fixed to the casing317b of the drum which has an inwardly turned apron 317c defining anopening 317d through which the suspension is introduced and solids areremoved, e.g. with the aid of the devices illustrated in FIG. 1 for thispurpose. The casing 317b, which is generally frustoconical, terminatesin a cylindrical extension 317e which carries an annular disk 327 whollyreceived with clearance in an outer housing structure 326. The lattercomprises a cylindrical inner wall 326a mounting on the rear surface ofplate 317 or formed by this plate. A radial annular flange 326b iscarried by the wall 326a and lies parallel to the disk 327 but isaxially offset to the right therefrom. Outwardly of the disk 327 andradially spaced therefrom, the outer housing portion is formed with acylindrical wall 326c which terminates in an apron 328, again lyingparallel to the disk 327 and axially spaced to the right thereof. Thewalls 328 and 326b flanking the disk 327 define, with the wall 326c,outer troughs 326d for which the edge 328a forms a discharge weir in themanner previously described, e.g. in connection with FIG. 3a. Inaddition, the plate 317, the wall 326a and the wall 326b define an innertrough 325 which communicates between the liquid-collecting compartment305 and the trough 326b behind the disk 327. This system createsautomatically the aforedescribed suction effect with the aid of a liquidcolumn.

Thus at the beginning of the filtration, only the collecting compartment305 and the annular space 307 are filled with liquid and, with theprogress of filtration, liquid passes over the inner edge 327a of thedisk 327 to drive air ahead of it and eventually force liquid into thetrough 326d and over the weir 328a. For this reason the edge 327a mustlie rearwardly outwardly of the level of the suspension medium withinthe drum at least at the inception of operation of the filtercentrifuge. In general terms, it has been found to be advantageous tolocate the edge 327a at the same radial level as the filter medium. Onceliquid passes the edge 328a, the siphon effect is created and continuesuntil the liquid-collecting compartment is fully drained. The trough326d acts as a trap preventing inflow of air and its output positionaugments the siphon effect as will immediately be apparent.

The discharge of air from the liquid-collecting compartment 405 can bepromoted by providing check valves 429 (FIG. 9) on the vertical-axisdrum immediately outwardly of the filter medium 403. In this case, theliquid-collecting compartment 405 can be filled with liquid through askimmer tube 408 extending into the discharge trough which communicatesvia ports 406 with the liquid-collecting compartment 405. Alternatively,a filling device such as that shown at 19 in FIG. 1 may also be used.The swingable skimmer tube 408 operates to control the level of theliquid in the outer trough as previously described.

In FIG. 10 there is shown a filter centrifuge drum 501 rotatable on itsshaft 502 and provided with a filter medium 503, e.g. a filter clothoverlying the perforated inner wall of this drum. The housing 520, as inthe embodiment of FIG. 7, is stationary and defines a siphon compartmentwith the drum which is provided with axially spaced plates 530 formingweir-type rings around which the liquid can pass. An outlet 506,disposed radially outwardly of the filter medium 503 communicates withthe liquid-collecting chamber 504 and is provided with a blocking valve503 while an overflow port 534 can be formed in the rear wall 532 of thehousing. Between the rings 530, the drum is provided withoutwardly-extending scoop-like vanes for entrainment of the liquid inthe collecting compartment in rotational movement.

At the beginning of the filtration process, the passage 506 is closed bythe valves 533. With rotation of the drum, the liquid traversing thefilter medium displaces air from the system until the liquid level inthe pockets outwardly of the drum reaches the level of port 534. At thispoint, valve 533 can be opened to lower the liquid level in the outercompartments and induce a hydrostatic suction in the manner previouslydescribed. The openings 506 must, for this purpose, lie radiallyoutwardly of the level of liquid within the drum and preferablyoutwardly of the filter medium.

The filter centrifuge of FIG. 11 operates generally in accordance withthe same principles but to reduce frictional engagement of the liquidwith the rotating drum or rotating liquid with the stationary housingwalls, an annular pocket 620 is formed inwardly of the ends of the drumwith a smaller volume than that defined between the rings 530. Theoutwardly-extending annular flanges 630 of the drum, however, areflanked by the walls of the pockets 620 so that the liquid must flowoutwardly around the flanges 630 and then over the walls of the pocket620 before reaching the outlet 634. A passage 606 is formed in the wallsof the pocket which defines the liquid-collecting compartment 605 and isprovided with the valve 633. The apparatus of FIG. 11, of course,operates in a manner similar to that of FIG. 10.

FIGS. 12 and 13 illustrate two embodiments of the invention which havebeen illustrated more or less diagrammatically in FIGS. 5 and 6.However, in place of the suction pump 64 of FIG. 6, where the pump islocated externally of the drum, the suction pumps of the embodiments ofFIGS. 12 and 13 are so-called jet or ejector pumps 838 and 839. In thissystem, the liquid flowing from the liquid-collecting compartment 805,805a under the hydrodynamic pressure of centrifugally displaced liquid,enters a conically constricted tube or passage 840 or 841 and is thenejected through a nozzle 842 or 843 into a further tube 844 or 845.

In FIG. 12, the passage 844 opens directly into the passage 806communicating with the outer trough-shaped compartment 807 while in FIG.13, the passage 805 opens into this compartment directly. In both cases,the jet pump operates as a venturi suction pump and a difference residesin the fact that all of the liquid passing through the outlet openings806a of the embodiment of FIG. 13 also traverses the pump whereas aportion of the liquid can pass via outlet 806 in the embodiment of FIG.12 into the compartment 807, thereby bypassing the pump. In theembodiment of FIG. 12, however, the liquid derives from an externalsource, e.g. a pipe 848.

Because of the reduction in cross-section of the nozzle 842 or 843, asuction is induced at the passages 846 and 847 to draw air from theliquid-collecting compartments. The pump of FIG. 12 may be used both toevacuate air from the system and to fill the system with liquid prior tofiltration. At the end of the filtration process, the same pump may beemployed to cause air to pass through the filter cake on the filtermedium 803. In the system of FIG. 13, however, the suction effectterminates as all liquid is driven from the system. To permit at leastpartial hydrodynamic suction to be generated, the ports communicatingwith the liquid-collecting compartment are extended as shown at 850 toproject below the surface of the perforated plate 801 so that some airmay be trapped at the outer surface of this plate.

FIG. 14 shows a filter centrifuge in which the liquid-collectingcompartment 905 is formed within a housing rotatable with and extendingaround the drum which carries the filter medium 903 in the mannerpreviously described. The perforated inner wall of the drum has tubularextensions 950 projecting into the liquid-collecting compartment and thelatter communicates with the atmosphere via a throttle device 951.

The throttle 951 comprises an outlet 906 having a conical configurationand a conical valve member 952 seated in this outlet. The valve member952 is connected by a rod 953 to a float 954 and is biased in an openingdirection by a spring 955 seated against a support 956. At a radiallyinner portion of the liquid-collecting compartment 905, a check valve929 is provided to permit escape of fluid from the compartment butprevent entry of fluid into the latter.

The suspension is introduced into the rotating drum 901 in whichcentrifugal force holds a valve 951 closed against the force of spring955 so that liquid collected in the compartment 905 cannot pass throughthis valve. The displaced air, however, is released through check valve929.

The liquid level grows progressively inwardly until it lifts the float954 to open the valve 954 and permit controlled outflow of liquid in themanner of a siphon. Should the liquid level fall excessively, thethrottle 951 likewise closes to permit buildup of the liquid level.Radially inwardly air inflow, however, is limited by the check valve. Asmall air cushion thus remains in the system without materiallyaffecting the operation as described. When the centrifuge is brought tostandstill, the spring 955 opens the valve 951 and permits liquid todrain from the system.

While reference has been made to air in the preceding description, itshould be noted that what was there said also applies to other gaseswhich may be used or generated.

We claim:
 1. A filter centrifuge comprising:a perforated drum rotatableabout an axis; means forming an annular chamber around said drum androtatable therewith; pump means for evacuating said chamber to withdrawair therefrom; means for rotating said drum about said axis to displacesaid liquid phase through said filter cake; and means for forming acontinuum consisting of liquid which radiates outwardly through saiddrum; said drum being adapted to form a filter cake through which aliquid phase can pass outwardly into said chamber with a hydrostaticpressure determined by the rate at which liquid of said continuum iswithdrawn from said chamber.
 2. The centrifuge defined in claim 1wherein said axis is vertical.
 3. The centrifuge defined in claim 1wherein the means for withdrawing liquid from said chamber includes anannular compartment surrounding said axis and communicating with saidchamber at a location radially outwardly of said drum.
 4. The centrifugedefined in claim 3 wherein said chamber is formed with an annular weirover which the liquid flows.
 5. The centrifuge defined in claim 4,further comprising an adjustable scoop receivable in said compartmentfor scooping liquid therefrom.
 6. The centrifuge defined in claim 4which includes a siphon arrangement draining liquid from said chamber.7. A method of operating a filter centrifuge comprising a perforateddrum rotatable about an axis and formed with a filter medium upon whicha filter cake can form during filtration, said drum being surrounded bya liquid-collecting compartment rotating together with said drum andhaving at least one outlet radially outwardly of said filter medium,said method comprising the steps of:rotating said drum about said axisat a rate sufficient to induce passage of a liquid phase of a suspensionoutwardly through said filter medium into said liquid-collectingcompartment; initially forming a liquid suction-pressure head withinsaid liquid-collecting compartment; withdrawing air from saidliquid-collecting compartment; and maintaining the liquidsuction-pressure head so as to produce a continuum consisting of liquid,which radiates outwardly through said drum, resulting from saidrotating.
 8. The method defined in claim 7 wherein liquid is drainedfrom said liquid-collecting compartment to apply said suction-pressurehead to the liquid at said filter medium by centrifugally inducing saidliquid to pass through said outlet into an annular trough surroundingsaid axis and rotatable with said drum, and scooping liquid out of saidtrough.